In power electronics, power semiconductor switches are often used to switch or control high magnitudes of voltage, current and/or power. Many different types of power semiconductor switches are known. Common power semiconductor switches include MOSFETs (metal oxide semiconductor field effect transistors), IGBTs (insulated gate bipolar transistor) and bipolar transistors. MOSFETs are advantageous in high power applications in that a low-voltage input signal can be used. Bipolar transistors are advantageous in that a high reverse voltage strength and a low forward voltage with a high current density may be achieved. These advantages of bipolar transistors are due to the so-called bipolar effect. The bipolar effect greatly increases specific conductivity of a semiconductor region in comparison with the intrinsic conductivity of the region that is defined by the basic doping. This is due to charge-neutral bipolar flooding of electrons and holes. IGBTs combine some of the advantages of unipolar MOSFETs and bipolar transistors. For this reason, IGBT power semiconductor devices are often used in power electronics applications as electronic switches, and in particular applications that require blocking of voltages of more than 200 Volts.
The development of IGBTs has resulted in considerable improvement in performance and, in particular, an increase in current densities of the IGBTs during normal operation. Furthermore, the level of bipolar charge carrier flooding (so-called plasma) has also continuously increased in IGBTs to optimize the on-state properties. A side-effect the increased plasma is that the intrinsic dynamics of evacuating plasma from the component dominates the voltage rise and the subsequent drop in the load current of the component under conventional drive conditions. As a result, the component's behavior deviates from quasi-steady state behavior (i.e. ideal transfer characteristics) determined by the MOS channel, particularly during switch-off. As a consequence, voltage and current transients are generally considerably steeper and therefore quicker during switch-on operations than during switch-off operations of the component.
Additionally, modern IGBT components are often designed to be limited by the pinch-off effect in the event of a short circuit to specific value, such as four times the nominal current density. This is done to prevent premature thermal destruction. This limitation in conjunction with a threshold voltage specific to the IGBT limits the gradient of the transfer characteristic curve. As a result, the position of the Miller plateau is greatly dependent on the switched load current of the component.
The above described switch-on properties of IGBTs are often a decisive factor in determining the electromagnetic compatibility (EMC) of an IGBT to a particular application.
Different methods for controlling the switching speed of semiconductor switches are known. For example, controlled current sources designed to provide defined phase sections or ramps for a temporal gate-emitter voltage profile and for a feedback of the gate voltage signal, and circuits designed to control the collector voltage or the collector current to the drive circuit are known for this purpose. These known methods are described in further detail in the following publications: DE 43 29 363 A1, EP 0 814 564 A1, JP2002300016 A, JP11069780 A, U.S. Pat. No. 6,271,709 B1, US 2006/0044025 A1, WO 94/23497, JP2002369553 A, U.S. Pat. No. 4,540,893 and DE 196 10 895 A1.
However, the switching behavior of modern components when driven by known driving methods is highly dependent on the selected operating point of the device, and in particular the load current to be switched.